Controller of internal combustion engine

ABSTRACT

An engine controller performs low opening degree control during a first intake stroke period since an engine start is commenced until first intake strokes of respective cylinders end. Thus, an opening degree of an intake throttle valve is controlled to a fully closed position or proximity of the fully closed position such that intake pressure downstream of the intake throttle valve becomes equal to or lower than critical pressure with respect to intake pressure upstream of the intake throttle valve during an intake stroke of each cylinder. The controller calculates a leak air quantity at the time when the intake throttle valve is fully closed based on an intake air quantity sensed during the low opening degree control. The controller corrects a feedback gain of idle speed control in accordance with the leak air quantity of the intake throttle valve.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Division of application Ser. No. 11/907,448, filedOct. 12, 2007, the entire contents of which are hereby incorporated byreference into this application. This application is also based on andclaims priority from Japanese Patent Applications No. 2006-286114 filedon Oct. 20, 2006 and No. 2006-286115 filed on Oct. 20, 2006, thecontents are hereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a controller of an internal combustionengine having intake throttle valves in intake passages of respectivecylinders of the engine for adjusting intake air quantities.

2. Description of Related Art

There has been a system having a throttle valve in an intake pipeupstream of intake manifolds of respective cylinders (i.e., in intakepipe collection part upstream of position where intake pipe branchesinto intake manifolds of cylinders) of an internal combustion engine foradjusting an intake air quantity and a bypass air quantity regulatingvalve (i.e., idle speed control valve) for adjusting a bypass airquantity flowing through a bypass passage bypassing the throttle valveto control idle speed. In such the system, there is a possibility that aleak air quantity of the throttle valve (air quantity passing throughsmall gap between throttle valve and inner wall surface of intakepassage when throttle valve is fully closed) varies due to manufacturetolerance, an aging change or the like and the controllability of theidle speed control decreases.

As a countermeasure, a device described in patent document 1(JP-A-H5-288101) performs fuel cut control when the throttle valve isfully closed and rotation speed of the engine is equal to or higher thana predetermined value and calculates the leak air quantity of thethrottle valve based on an intake air quantity sensed with an airflowmeter while the bypass air quantity regulating valve is fully closedduring the fuel cut control (i.e., while throttle valve is fullyclosed). The device controls the bypass air quantity regulating valvewith the use of the leak air quantity during the idle operation.

When a device described in patent document 2 (JP-A-H9-170474) performsfeedback control of the bypass air quantity regulating valve to conformactual rotation speed to target idle speed during the idle operation ofthe engine, the device estimates an external load of the engine andsubtracts a control amount corresponding to the external load from afeedback correction amount. Thus, the device obtains and learns a valuecorresponding to a change of the leak air quantity of the throttle valveand corrects the feedback correction amount by using the learning value.

The applicants of the present application are currently studying asystem having intake throttle valves in intake manifolds of respectivecylinders of an internal combustion engine for adjusting intake airquantities. In such the system, as shown in FIG. 3, specifically in anarea of a low opening degree Thr of the intake throttle valve (forexample, in idle operation area), the quantity Gath of the passing airof the intake throttle valve increases and the intake air quantityincreases as the leak air quantity Qleak of the intake throttle valve(air quantity passing through gap between intake throttle valve andinner wall surface of intake passage when intake throttle valve is fullyclosed) increases even when the opening degree Thr of the intakethrottle valve is the same. Accordingly, there is a possibility that therotation of the engine rises during the idle operation.

If the leak air quantity Qleak of the intake throttle valve decreases,the passing air quantity Gath of the intake throttle valve decreases andthe intake air quantity decreases even when the opening degree Thr ofthe intake throttle valve is the same. Therefore, there is a possibilitythat the rotation of the engine falls.

Moreover, if the leak air quantity Qleak of the intake throttle valvechanges, the relationship between the opening degree Thr of the intakethrottle valve and the passing air quantity Gath (i.e., changecharacteristic of passing air quantity Gath with respect to openingdegree Thr of intake throttle valve) changes. Therefore, there occurs aproblem that the control accuracy of the intake air quantity by theopening degree control of the intake throttle valve lowers.

A following problem will occur if the leak air quantity of the intakethrottle valve is calculated based on the intake air quantity sensedwith the airflow meter while the intake throttle valve is fully closedduring the fuel cut control by using the technology of the patentdocument 1 in the system having the intake throttle valves in the intakemanifolds of the respective cylinders of the engine. That is, thecapacity of the intake passage downstream of the intake throttle valveis small in the system having the intake throttle valves in the intakemanifolds of the respective cylinders of the engine. Therefore, if theintake throttle valve is fully closed during the fuel cut control (i.e.,when rotation speed of engine is equal to or higher than fuel cutresuming rotation speed), intake air pressure downstream of the intakethrottle valve declines greatly. As a result, there is a possibilitythat oil loss via valve guides (i.e., phenomenon that oil lubricatingsliding parts of intake valve or the like leaks toward intake port andis suctioned into intake port) occurs and the combustion state and theemission of the engine worsen.

When the feedback control of the intake throttle valve is performed toconform the actual rotation speed to the target idle speed during theidle operation of the engine with the use of the technology of thepatent document 2 in the system having the intake throttle valves in theintake manifolds of the respective cylinders of the engine, a method ofcalculating a value corresponding to the leak air quantity of the intakethrottle valve by estimating the external load of the engine and byremoving the control amount corresponding to the external load from thefeedback correction amount could be employed. However, it is difficultto estimate the external load of the engine with high accuracy.Therefore, the method of calculating the leak air quantity of the intakethrottle valve based on the feedback correction amount and the externalload has a defect that the leak air quantity of the intake throttlevalve cannot be calculated with high accuracy due to an estimation errorof the external load.

A system described in patent document 3 (Japanese Patent Gazette No.2536242) has shutoff valves (i.e., throttle valves) in intake passagesof respective cylinders of an internal combustion engine for adjustingintake air quantities respectively and bypass passages bypassing theshutoff valves. The system has control valves (i.e., idle speed controlvalves) in the bypass passages of the respective cylinders foropening/closing the bypass passages respectively. During the idleoperation period, the system fully closes the shutoff valves provided inthe intake passages of the respective cylinders and controls valveopening periods of the control valves provided in the bypass passages ofthe cylinders. Thus, the system adjusts the intake air quantities andthe idle speed.

In such the system, even if the valve opening periods of the controlvalves provided in the bypass passages of the respective cylinders areequalized during the idle operation, a variation is caused among theintake air quantities of the cylinders if the leak air quantities of theshutoff valves provided in the intake passages of the cylinders (airquantities passing through small gaps between shutoff valves and intakepassage inner walls when shutoff valves are fully closed) vary among thecylinders due to manufacture tolerances, aging changes, and the like.Therefore, there is a possibility that torque of the respectivecylinders varies and the idle speed fluctuate largely.

As a countermeasure, a technology described in the patent document 3senses the rotation speed as of expansion strokes of the respectivecylinders during the idle operation and calculates average rotationspeed of all the cylinders. The technology corrects the valve openingperiod of each control valve provided in each bypass passage of eachcylinder in accordance with a difference between the rotation speed ofthe cylinder and the average rotation speed of all the cylinders.

This technology corrects the valve opening period of each control valveprovided in the bypass passage of each cylinder during the idleoperation, in which the shutoff valve provided in the intake passage ofeach cylinder is fully closed. Thus, the technology corrects thevariation among the intake air quantities of the cylinders due to thevariation among the leak air quantities of the shutoff valves of therespective cylinders or the like during the idle operation. Therefore,in the operation range, in which the shutoff valves provided in theintake passages of the cylinders are opened, the variation among theintake air quantities of the cylinders due to the variation among theleak air quantities of the shutoff valves of the cylinders cannot becorrected. Accordingly, the rotation fluctuation of the engine due tothe variation among the leak air quantities of the shutoff valves of thecylinders cannot be inhibited.

Moreover, when the technology of the patent document 3 is applied,installation of the bypass passages to the intake passages of therespective cylinders and installation of the control valves in thebypass passages of the respective cylinders are necessary. Therefore,the system structure will be complicated and the cost will be increased.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a controller of aninternal combustion engine with a system having intake throttle valvesin intake passages of respective cylinders of the engine capable ofcalculating leak air quantities of the intake throttle valves of therespective cylinders with high accuracy and of improving controllabilityof the intake air quantities without posing adverse effects to operationof the engine.

It is another object of the present invention to provide a controller ofan internal combustion engine with a system having intake throttlevalves in intake passages of respective cylinders of the engine capableof correcting a variation among intake air quantities due to a variationamong leak air quantities of the intake throttle valves of therespective cylinders with high accuracy, suppressing rotationfluctuation of the engine due to the variation among the leak airquantities of the intake throttle valves of the respective cylinders andsatisfying request for cost reduction.

According to an aspect of the present invention, a controller of aninternal combustion engine having branch intake passages, which branchfrom a main intake passage of the engine and introduce intake air intorespective cylinders, and intake throttle valves in the branch intakepassages of the respective cylinders for adjusting the intake quantitieshas an intake air quantity sensor provided in the main intake passagefor sensing the intake air quantity. The controller has a low openingdegree control device that performs low opening degree control forcontrolling an opening degree of the intake throttle valve during afirst intake stroke period since an engine start is commenced untilfirst intake strokes of the respective cylinders are completed such thatintake pressure downstream of the intake throttle valve becomes pressureequal to or lower than predetermined critical pressure with respect tointake pressure upstream of the intake throttle valve during an intakestroke of each cylinder. The controller has a leak air quantitycalculation device that calculates a leak air quantity at the time whenthe intake throttle valve is fully closed based on the intake airquantity sensed with the intake air quantity sensor during the lowopening degree control. The controller has an intake throttle valveopening degree correction device that corrects the opening degree of theintake throttle valve in accordance with the leak air quantity.

With this structure, the low opening degree control for controlling theopening degree of the intake throttle valve to the fully closed positionor proximity of the fully closed position is performed so that theintake pressure downstream of the intake throttle valve becomes pressure(pressure at which passing air quantity changes in accordance withopening degree of intake throttle valve without being affected bypressure difference between pressure upstream of intake throttle valveand pressure downstream of intake throttle valve) equal to or lower thanthe predetermined critical pressure with respect to the intake pressureupstream of the intake throttle valve. By sensing the intake airquantity during the low opening degree control, the passing air quantitycorresponding to the opening degree of the intake throttle valve duringthe low opening degree control can be sensed. During the low openingdegree control, the passing air quantity changes in accordance with theopening degree of the intake throttle valve without being affected bythe pressure difference between the pressure upstream of the intakethrottle valve and the pressure downstream of the intake throttle valve.Therefore, by using a map or the like beforehand storing therelationship between the opening degree of the intake throttle valve andthe passing air quantity during the low opening degree control in theform of data, the leak air quantity as the passing air quantity at thetime when the intake throttle valve is fully closed can be calculatedwith high accuracy from the intake air quantity sensed with the intakeair quantity sensor during the low opening degree control, i.e., thepassing air quantity corresponding to the opening degree of the intakethrottle valve under the low opening degree control.

The air is stored in the intake passage downstream of the intakethrottle valve before the first intake strokes of the cylinders arecompleted after the engine start is commenced. Therefore, even if thelow opening degree control for controlling the opening degree of theintake throttle valve to the fully closed position or the proximity ofthe fully closed position is performed during the first intake strokeperiod of the cylinders since the engine start is commenced until thefirst intake strokes of the respective cylinders are completed, the airnecessary for the combustion in the engine start can be taken into thecylinders. As a result, adverse effect on the starting performance ofthe engine can be inhibited.

The change of the relationship between the opening degree of the intakethrottle valve and the passing air quantity due to the change of theleak air quantity of the intake throttle valve (change characteristic ofpassing air quantity with respect to opening degree of intake throttlevalve) can be compensated by correcting the opening degree of the intakethrottle valve in accordance with the calculated leak air quantity ofthe intake throttle valve. Accordingly, the controllability of theintake air quantity through the opening degree control of the intakethrottle valve can be improved without being affected by the agingchange of the leak air quantity of the intake throttle valve and thelike.

Furthermore, the leak air quantity of the intake throttle valve can becalculated when the engine is started. Therefore, the opening degree ofthe intake throttle valve can be corrected in accordance with the leakair quantity of the intake throttle valve even immediately after theengine start. Thus, the controllability of the intake air quantity canbe improved even immediately after the engine start.

According to another aspect of the present invention, the controllerperforms low opening degree control for controlling the opening degreeof the intake throttle valve during fuel cut control for stopping fuelinjection of the engine so that the intake pressure downstream of theintake throttle valve becomes pressure that is equal to or lower thanthe predetermined critical pressure with respect to the intake pressureupstream of the intake throttle valve and that does not cause oil lossvia valve guides in the intake stroke of each cylinder. The controllercalculates a leak air quantity at the time when the intake throttlevalve is fully closed based on the intake air quantity sensed with theintake air quantity sensor during the low opening degree control andcorrects the opening degree of the intake throttle valve in accordancewith the leak air quantity.

In the system having the intake throttle valves in the intake passagesof the respective cylinders of the engine, the capacity of the intakepassage downstream of the intake throttle valve is small. Therefore, ifthe intake throttle valve is fully closed during the fuel cut control(i.e., when rotation speed of engine is equal to or higher than fuel cutresuming rotation speed), there is a possibility that the intakepressure downstream of the intake throttle valve declines greatly andthe oil loss via the valve guides occurs. The above-described controllerperforms the low opening degree control for controlling the openingdegree of the intake throttle valve to the fully closed position orproximity of the fully closed position so that the intake pressuredownstream of the intake throttle valve becomes the pressure that isequal to or lower than the critical pressure with respect to the intakepressure upstream of the intake throttle valve and that does not causethe oil loss via the valve guides during the fuel cut control. Thecontroller calculates the leak air quantity at the time when the intakethrottle valve is fully closed based on the intake air quantity sensedwith the intake air quantity sensor during the low opening degreecontrol. Thus, the leak air quantity of the intake throttle valve can becalculated with high accuracy while inhibiting the oil loss via thevalve guides and eventual deterioration of the combustion state oremission of the engine.

According to yet another aspect of the present invention, a controllerof an internal combustion engine having intake throttle valves in intakepassages of respective cylinders of the engine for regulating intake airquantities, each intake throttle valve having a function to generate anairflow for equalizing a fuel-air mixture, has an exhaust gasrecirculation adjustment device that adjusts an exhaust gasrecirculation quantity of the engine, an exhaust gas recirculationincrease control device that performs exhaust gas recirculation increasecontrol for controlling the exhaust gas recirculation adjustment devicesuch that the quantity of the exhaust gas recirculation increases duringlow load operation of the engine, an each cylinder leak air quantityinformation sensing device that senses a combustion state in eachcylinder during the exhaust gas recirculation increase control asinformation about the leak air quantity at the time when the intakethrottle valve of each cylinder is fully closed, a large leak aircylinder determination device that determines a cylinder (large leak aircylinder) causing a large leak air quantity of the intake throttle valvebased on the sensed combustion state in each cylinder, and an eachcylinder intake throttle valve opening degree correction device thatcorrects the opening degree of the intake throttle valve during a periodcorresponding to an intake stroke of the large leak air cylinder inaccordance with the combustion state in the large leak air cylinder.

If the leak air quantity of the intake throttle valve increases when theintake throttle valve has the function to generate the airflow (e.g.,tumble flow or swirl flow) for equalizing the fuel-air mixture,intensity of the airflow generated by the intake throttle valve isweakened correspondingly and the effect to equalize the fuel-air mixtureis lowered. Therefore, if the exhaust gas recirculation quantity (EGRquantity) is increased during the low load operation of the engine, inwhich the influence of the EGR is large, the equalizing effect of thefuel-air mixture is further lowered by the influence of the EGR and thecombustion state becomes unstable in the cylinder corresponding to theintake throttle valve with the large leak air quantity.

Paying attention to such the characteristic, the EGR increase controlfor controlling the EGR adjustment device to increase the EGR quantityduring the low load operation of the engine is performed. The combustionstates of the respective cylinders are sensed as information about theleak air quantities of the intake throttle valves of the respectivecylinders during the EGR increase control, and the cylinder causing theunstable combustion state is determined based on the combustion statesof the respective cylinders. Thus, the large leak air cylinder (cylinderwith large leak air quantity) can be determined with high accuracy. Thecontroller corrects the opening degree of the intake throttle valveduring the period corresponding to the intake stroke of the large leakair cylinder in accordance with the combustion state (information aboutleak air quantity) in the large leak air cylinder. Thus, the openingdegree of the intake throttle valve can be corrected in accordance withthe leak air quantity of the intake throttle valve of the large leak aircylinder. By repeatedly performing the processing, the variation amongthe intake air quantities due to the variation among the leak airquantities of the intake throttle valves of the respective cylinders canbe corrected with high accuracy, and the rotation fluctuation of theengine due to the variation among the leak air quantities of the intakethrottle valves of the respective cylinders can be suppressed.

Moreover, there is no need to provide bypass passages bypassing theintake throttle valves of the respective cylinders or control valves foropening/closing the bypass passages of the respective cylinders.Therefore, the system structure can be simplified and the cost can bereduced.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of embodiments will be appreciated, as well asmethods of operation and the function of the related parts, from a studyof the following detailed description, the appended claims, and thedrawings, all of which form a part of this application. In the drawings:

FIG. 1 is a schematic diagram showing an engine control system accordingto a first embodiment of the present invention;

FIG. 2 is a longitudinal cross-sectional view showing an intake throttlevalve unit and a proximity thereof according to the first embodiment;

FIG. 3 is a diagram showing a relationship between a leak air quantityand an opening degree of the intake throttle valve;

FIG. 4 is a time chart for explaining a calculation method of the leakair quantity according to the first embodiment;

FIG. 5 is a diagram showing a pressure range equal to or lower thancritical pressure;

FIG. 6 is a flowchart showing a processing flow of a leak air quantitycalculation program according to the first embodiment;

FIG. 7 is a flowchart showing a processing flow of ISC feedbackcorrection amount calculation program according to the first embodiment;

FIG. 8 is a map showing a basic leak air quantity according to the firstembodiment;

FIG. 9 is a map showing a relationship between the opening degree of theintake throttle valve and a passing air quantity during low openingdegree control according to the first embodiment;

FIG. 10 is a time chart for explaining a calculation method of a leakair quantity according to a second embodiment of the present invention;

FIG. 11 is a flowchart showing a processing flow of a leak air quantitycalculation program according to the second embodiment;

FIG. 12 is a schematic diagram showing an engine control systemaccording to a third embodiment of the present invention;

FIG. 13 is a time chart for explaining each cylinder intake throttlevalve opening degree correction according to a third embodiment of thepresent invention; and

FIG. 14 is a flowchart showing a processing flow of an each cylinderintake throttle valve opening degree correction program according to thethird embodiment.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

A first embodiment of the present invention will be explained inreference to FIGS. 1 to 9. First, a schematic structure of an engineintake system will be explained with reference to FIG. 1. An engine 11(for example, inline four-cylinder engine) as an internal combustionengine has four cylinders of a first cylinder #1 to a fourth cylinder#4. An airflow meter 23 (intake air quantity sensor) that senses anintake air quantity is provided in an intake pipe 12 (main intakepassage) of the engine 11. A surge tank 13 is provided downstream of theairflow meter 23, and intake manifolds 14 (branch intake passages) forintroducing air into respective cylinders of the engine 11 are providedto the surge tank 13. Intake throttle valve units 15 are attached to theintake manifolds 14 of the respective cylinders, and injectors (notshown) for injecting fuel are attached near intake ports of therespective cylinders. Spark plugs (not shown) are attached to a cylinderhead of the engine 11 for the respective cylinders. A fuel air mixturein the cylinders is ignited with spark discharge from the respectivespark plugs.

A coolant temperature sensor 25 for sensing coolant temperature Tw and acrank angle sensor 26 for outputting a pulse signal every time acrankshaft of the engine 11 rotates by a predetermined crank angle areattached to a cylinder block of the engine 11. A crank angle CA andengine rotation speed Ne are sensed based on the output signal of thecrank angle sensor 26. An accelerator sensor 27 senses an acceleratoroperation amount ACCP (depressed amount of accelerator).

Next, the structure of the intake throttle valve unit 15 will beexplained in reference to FIG. 2. In the intake throttle valve unit 15of each cylinder, an intake passage 18 with a substantially quadrangularcross-section is formed in a housing 17 made of a resin. An intakethrottle valve 19 of a cantilever type for opening and closing theintake passage 18 is provided in the intake passage 18. A shaft 20 as arotary shaft is attached to a lower end portion of the intake throttlevalve 19 such that the intake throttle valve 19 can rotate about theshaft 20 in an opening direction and a closing direction. Each intakethrottle valve 19 is formed in the shape that matches with thecross-sectional shape of the intake passage 18 (i.e., substantiallyquadrangular shape in the present embodiment). The cross-sectional shapeof the intake passage 18 or the shape of the intake throttle valve 19 isnot limited to the substantially quadrangular shape. Rather, thecross-sectional shape or the shape may be any other shape such assubstantially a semicircular shape or substantially a half-ellipticalshape.

The intake throttle valves 19 of the respective cylinders are connectedto the common shaft 20 and rotate integrally. A motor 21 (shown inFIG. 1) connected to the shaft 20 is controlled in accordance with anengine operation condition (accelerator operation amount ACCP and thelike) to control an opening degree Thr of the intake throttle valves 19of the cylinders. The opening degree Thr of the intake throttle valves19 is sensed with an intake throttle valve opening degree sensor 29(shown in FIG. 1).

The intake throttle valve 19 of each cylinder is provided such that anend (lower end) on the shaft 20 side thereof contacts (or is locatednear) an inner wall face of the housing 17 and such that the intake aircan hardly pass under the intake throttle valve 19. When the intakethrottle valve 19 is opened, a flow passage (gap between intake throttlevalve 19 and inner wall face of housing 17) of the intake air is formedonly above the intake throttle valve 19 and a flow passagecross-sectional area above the intake throttle valve 19 changes inaccordance with the opening degree Thr of the intake throttle valve 19.During low load operation of the engine 11, in which the opening degreeThr of the intake throttle valve 19 becomes comparatively small, anairflow (for example, tumble flow or swirl flow) for equalizing thefuel-air mixture in the cylinder can be generated by passing the intakeair only through the upper portion of the intake passage 18 and byincreasing flow velocity of the intake air. An accommodation recess 22for accommodating the intake throttle valve 19 when the intake throttlevalve 19 is fully opened is formed in the housing 17 and a neighborhoodthereof such that the intake throttle valve 19 does not hinder theintake air flow when the intake throttle valve 19 is fully opened.

Outputs of the above-described various sensors are inputted into acontrol circuit 28 (ECU). The ECU 28 is constituted mainly by amicrocomputer. The ECU 28 controls a fuel injection quantity of theinjector and ignition timing of the spark plug in accordance with theengine operation state by executing various kinds of engine controlprograms stored in an incorporated ROM (storage medium).

Furthermore, the ECU 28 calculates a target opening degree of the intakethrottle valve 19 based on the accelerator operation amount ACCP sensedwith the accelerator sensor 27 and the like and controls the motor 21 ofthe intake throttle valve 19 to coincide the actual opening degree Throf the intake throttle valve 19 with the target opening degree.

As shown in FIG. 3, specifically in the low opening degree range of theintake throttle valve 19 (for example, in idle operation range), if aleak air quantity Qleak of the intake throttle valve 19 increases, apassing air quantity Gath of the intake throttle valve 19 can increaseand the intake air quantity can increase even when the opening degreeThr of the intake throttle valve 19 is the same. The leak air quantityQleak is an air quantity passing through the gap between the intakethrottle valve 19 and the inner wall face of the intake passage when theintake throttle valve 19 is fully closed. Therefore, there is apossibility that the rotation of the engine 11 rises during the idleoperation. If the leak air quantity Qleak of the intake throttle valve19 decreases, the passing air quantity Gath of the intake throttle valve19 decreases and the intake air quantity decreases even when the openingdegree Thr of the intake throttle valve 19 is the same. Therefore, thereis a possibility that the rotation of the engine falls. Moreover, if theleak air quantity Qleak of the intake throttle valve 19 changes, therelationship between the opening degree Thr of the intake throttle valve19 and the passing air quantity Gath (change characteristic of passingair quantity Gath with respect to opening degree Thr of intake throttlevalve 19) changes. Accordingly, there is a problem that the controlaccuracy of the intake air quantity by the opening degree control of theintake throttle valve 19 falls.

As a countermeasure, the ECU 28 first executes a leak air quantitycalculation program shown in FIG. 6 to calculate the leak air quantityQleak of the intake throttle valve 19 as follows. As shown in a timechart of FIG. 4, low opening degree control is performed during a firstintake stroke period A since the engine start (START) is commenced untilthe first intake strokes of the cylinders end. The low opening degreecontrol is for controlling the opening degree Thr of the intake throttlevalve 19 to a certain opening degree for the low opening degree control(fully closed position or proximity of fully closed position) such thatthe pressure downstream of the intake throttle valve 19 becomes pressureequal to or lower than predetermined critical pressure CP with respectto the intake pressure upstream of the intake throttle valve 19, i.e.,pressure at which the passing air quantity Gath changes in accordancewith the opening degree Thr of the intake throttle valve 19 withoutbeing affected by the pressure difference between the pressure upstreamof the intake throttle valve 19 and the pressure downstream of theintake throttle valve 19, during the intake stroke of each cylinder. InFIG. 4, P#1-P#4 represent the intake pressure of the first to fourthcylinders #1-#4 respectively, and FL represents a low opening degreecontrol execution flag. I, C, E1 and E2 represent the intake stroke, acompression stroke, an expansion stroke and an exhaustion stroke of eachcylinder respectively.

Next, a setting method of the opening degree for the low opening degreecontrol, i.e., the opening degree at which the intake pressuredownstream of the intake throttle valve 19 becomes the pressure equal toor lower than the critical pressure CP with respect to the intakepressure upstream of the intake throttle valve 19, will be explained. Infollowing Formula (1) of an orifice, if fcom(Pim/Pamb) becomes constant,the passing air quantity Gath of the intake throttle valve 19 changes inaccordance with the opening degree (effective flow passagecross-sectional area Aeff) of the intake throttle valve 19, withoutbeing affected by the pressure difference between the intake pressurePamb upstream of the intake throttle valve 19 and the intake pressurePim downstream of the intake throttle valve 19. In Formula (1), Cthrepresents the flow rate coefficient, R is the gas constant, and T isintake temperature.

$\begin{matrix}{{Gath} = {{Cth} \times \frac{{Aeff} \times {Pamb}}{\sqrt{{R \times T}\;}} \times {{fcom}\left( {{Pim}/{Pamb}} \right)}}} & {{Formula}\mspace{20mu}(1)}\end{matrix}$

Therefore, in the relationship (shown in FIG. 5) between Pim/Pamb andfcom(Pim/Pamb) defined by following Formulas (2), (3) of an isentropicflow, a range B shown in FIG. 5 where fcom(Pim/Pamb) is constant is arange where the intake pressure Pim downstream of the intake throttlevalve 19 becomes the pressure equal to or lower than the criticalpressure CP with respect to the intake pressure Pamb upstream of theintake throttle valve 19. κ in Formulas (2), (3) is the specific heatratio.

$\begin{matrix}{{{{fcom}\left( {{Pim}/{Pamb}} \right)} = \sqrt{\kappa \times \left( \frac{2}{\kappa + 1} \right)^{\frac{\kappa + 1}{\kappa - 1}}}}{{{when}\mspace{20mu}{Pim}} \leq {\left( \frac{2}{\kappa + 1} \right)^{\frac{\kappa}{\kappa - 1}} \times {Pamb}}}} & {{Formula}\mspace{14mu}(2)} \\{{{{fcom}\left( {{Pim}/{Pamb}} \right)} = \sqrt{\frac{2 \times \kappa}{\kappa - 1}\left( {\left( \frac{Pim}{Pamb} \right)^{\frac{2}{\kappa}} - \left( \frac{Pim}{Pamb} \right)^{\frac{\kappa + 1}{\kappa}}} \right)}}{{{when}\mspace{20mu}{Pim}} > {\left( \frac{2}{\kappa + 1} \right)^{\frac{\kappa}{\kappa - 1}} \times {Pamb}}}} & {{Formula}\mspace{20mu}(3)}\end{matrix}$

Therefore, the opening degree for the low opening degree control can beset by calculating the opening degree Thr of the intake throttle valve19 that satisfies following Formula (4) as a condition that makesfcom(Pim/Pamb) constant, i.e., a condition that satisfies Formula (2) ofthe isentropic flow.

$\begin{matrix}{{Pim} < {\left( \frac{2}{\kappa + 1} \right)^{\frac{\kappa}{\kappa - 1}} \times {Pamb}}} & {{Formula}\mspace{14mu}(4)}\end{matrix}$

The intake air quantity is sensed with the airflow meter 23 during theexecution of the low opening degree control for controlling the openingdegree Thr of the intake throttle valve 19 to the opening degree for thelow opening degree control. Based on the intake air quantity sensed withthe airflow meter 23, the leak air quantity Qleak at the time when theintake throttle valve 19 is fully closed is calculated.

Thus, the passing air quantity Gath corresponding to the opening degreeThr of the intake throttle valve 19 during the low opening degreecontrol can be sensed by sensing the intake air quantity with theairflow meter 23 during the low opening degree control. During the lowopening degree control, the passing air quantity Gath changes inaccordance with the opening degree of the intake throttle valve 19,without being affected by the pressure difference between the pressureupstream of the intake throttle valve 19 and the pressure downstream ofthe intake throttle valve 19. Therefore, by using a map or the likebeforehand storing the relationship between the opening degree of theintake throttle valve 19 and the passing air quantity Gath during thelow opening degree control, the leak air quantity Qleak as the passingair quantity Gath at the time when the intake throttle valve 19 is fullyclosed can be calculated with high accuracy from the intake air quantitysensed with the airflow meter 23 during the low opening degree control,i.e., the passing air quantity Gath corresponding to the opening degreeThr of the intake throttle valve 19 for the low opening degree control.

Furthermore, the ECU 28 executes an ISC (idling speed control) feedbackcorrection amount calculation program shown in FIG. 7 to calculate anISC feedback correction amount ISCI for conforming actual enginerotation speed to target idle speed when a predetermined ISC executioncondition is satisfied. The ECU 28 controls the motor 21 of the intakethrottle valve 19 with the use of the ISC feedback correction amountISCI to perform ISC (idle speed control) for feedback-controlling theopening degree Thr of the intake throttle valve 19.

The ECU 28 corrects an integration amount ΔI (feedback gain of ISC) ofthe ISC feedback correction amount ISCI in accordance with the leak airquantity Qleak of the intake throttle valve 19 to correct the openingdegree Thr of the intake throttle valve 19 in accordance with the leakair quantity Qleak of the intake throttle valve 19. Thus, the change inthe relationship between the opening degree Thr of the intake throttlevalve 19 and the passing air quantity Gath (change characteristic ofpassing air quantity Gath with respect to opening degree Thr of intakethrottle valve 19) due to the change of the leak air quantity Qleak ofthe intake throttle valve 19 is compensated to improve stability of theidle speed.

Next, contents of processing of the leak air quantity calculationprogram of FIG. 6 and the ISC feedback correction amount calculationprogram of FIG. 7 executed by the ECU 28 will be explained. The leak airquantity calculation program shown in FIG. 6 is executed in apredetermined cycle while the ECU 28 is energized. If the program isstarted, first, S101 determines whether the engine start is in progress.If S101 is YES, the processing proceeds to S102. S102 determines whetherthe present time is in the first intake stroke period since the enginestart is commenced until the first intake strokes of the respectivecylinders are completed.

If S102 is YES, the processing proceeds to S103 to set the targetopening degree TThr of the intake throttle valve 19 at the openingdegree LThr for the low opening degree control. The opening degree LThrfor the low opening degree control is the opening degree Thr with whichthe intake pressure downstream of the intake throttle valve 19 becomesthe pressure equal to or lower than the critical pressure CP withrespect to the intake pressure upstream of the intake throttle valve 19(i.e., pressure at which passing air quantity Gath changes in accordancewith opening degree of intake throttle valve 19 without being affectedby pressure difference between pressure upstream of intake throttlevalve 19 and pressure downstream of intake throttle valve 19) during theintake stroke of each cylinder.

S103 sets the target opening degree TThr of the intake throttle valve 19at the opening degree LThr for the low opening degree control to controlthe actual opening degree Thr of the intake throttle valve 19 to theopening degree LThr for the low opening degree control (target openingdegree TThr). Thus, the low opening degree control for controlling theopening degree of the intake throttle valve 19 so that the intakepressure downstream of the intake throttle valve 19 becomes the pressureequal to or lower than the critical pressure CP with respect to theintake pressure upstream of the intake throttle valve 19 in the intakestroke of each cylinder is performed.

Then, the processing proceeds to S104 to read the intake air quantityQafm, the engine rotation speed Ne, the coolant temperature Tw and theactual opening degree Thr of the intake throttle valve 19 sensed withthe airflow meter 23, the crank angle sensor 26, the coolant temperaturesensor 25 and the intake throttle valve opening degree sensor 29 duringthe low opening degree control, and the like.

Then, the processing proceeds to S105 to calculate a basic leak airquantity Qleakbse at the time when the intake throttle valve 19 is fullyclosed in accordance with the intake air quantity Qafm and the actualopening degree Thr of the intake throttle valve 19 of the present time(i.e., during low opening degree control) with reference to a map of thebasic leak air quantity Qleakbse shown in FIG. 8. The map of the basicleak air quantity Qleakbse shown in FIG. 8 is set based on therelationship (in range C shown in FIG. 9) between the opening degree Throf the intake throttle valve 19 and the passing air quantity Gath duringthe low opening degree control (i.e., in state where passing airquantity Gath changes in accordance with opening degree of intakethrottle valve 19 without being affected by pressure difference betweenpressure upstream of intake throttle valve 19 and pressure downstream ofintake throttle valve 19) beforehand obtained based on test data, designdata and the like. For example, the map of the basic leak air quantityQleakbse is set such that the basic leak air quantity Qleakbse increasesas the intake air quantity Qafm during the low opening degree controlincreases and the basic leak air quantity Qleakbse increases as theactual opening degree Thr of the intake throttle valve 19 during the lowopening degree control decreases.

Then, the processing proceeds to S106 to calculate a correctioncoefficient Cne corresponding to the engine rotation speed Ne and thecoolant temperature Tw with reference to a map of the correctioncoefficient Cne (not shown). The map of the correction coefficient Cneis set based on the relationship between the engine rotation speed Neand the leak air quantity Qleak at the time when the intake throttlevalve 19 is fully closed and the relationship between the coolanttemperature Tw and the leak air quantity Qleak at the time when theintake throttle valve 19 is fully closed, which are beforehand obtainedbased on test data, design data and the like.

Then, the processing proceeds to S107 to calculate the leak air quantityQleak at the time when the intake throttle valve 19 is fully closed bymultiplying the basic leak air quantity Qleakbse at the time when theintake throttle valve 19 is fully closed by the correction coefficientCne (i.e., Qleak=Qleakbse×Cne).

Then, the processing proceeds to S108. S108 updates the learning valueof the leak air quantity Qleak in a learning area corresponding to thecoolant temperature Tw as of the calculation of the present leak airquantity Qleak with the presently calculated leak air quantity Qleak andstores the learning value in a rewritable nonvolatile memory such as abackup RAM (not shown) of the ECU 28.

The ISC feedback correction amount calculation program shown in FIG. 7is executed in a predetermined cycle while the ECU 28 is energized. Ifthe program is started, S201 first determines whether the ISC executioncondition is satisfied, for example, based on whether all conditionsincluding a condition that the intake throttle valve 19 is fully closed,a condition that vehicle speed is equal to or lower than a predeterminedvalue and a condition that the engine rotation speed Ne is within apredetermined range are satisfied.

If S201 determines that the ISC execution condition is satisfied, theprocessing proceeds to S202 to read the actual engine rotation speed Nesensed with the crank angle sensor 26. Then, the processing proceeds toS203 to calculate target idle speed Ns corresponding to the presentcoolant temperature Tw with reference to a map of the target idle speedNs (not shown).

Then, the processing proceeds to S204 to correct the integration amountΔI of the ISC feedback correction amount ISCI in accordance with theleak air quantity Qleak at the time when the intake throttle valve 19 isfully closed. In this case, the integration amount ΔI is corrected tocorrect the change in the relationship between the opening degree Thr ofthe intake throttle valve 19 and the passing air quantity Gath (changecharacteristic of passing air quantity with respect to opening degreeThr of intake throttle valve) due to the change of the leak air quantityQleak of the intake throttle valve 19.

Then, the processing proceeds to S205 to compare the actual enginerotation speed Ne and the target idle speed Ns. If it is determined thatthe actual engine rotation speed Ne is lower than the target idle speedNs, the processing proceeds to S206 to perform the correction forincreasing the ISC feedback correction amount ISCI by the integrationamount ΔI (i.e., ISCI=ISCI+ΔI).

If it is determined that the actual engine rotation speed Ne is higherthan the target idle speed Ns, the processing proceeds to S207 toperform correction for decreasing the ISC feedback correction amountISCI by the integration amount ΔI (i.e., ISCI=ISCI−ΔI).

The above-described first embodiment performs the low opening degreecontrol for controlling the opening degree of the intake throttle valve19 to the fully closed position or the proximity of the fully closedposition during the first intake stroke period since the engine start iscommenced until the first intake strokes of the respective cylinders arecompleted. Thus, the intake pressure downstream of the intake throttlevalve 19 is brought to the pressure equal to or lower than the criticalpressure with respect to the intake pressure upstream of the intakethrottle valve 19, i.e., the pressure at which the passing air quantitychanges in accordance with the opening degree of the intake throttlevalve 19 without being affected by the pressure difference between thepressure upstream of the intake throttle valve 19 and the pressuredownstream of the intake throttle valve 19. The leak air quantity as thepassing air quantity at the time when the intake throttle valve 19 isfully closed is calculated based on the intake air quantity sensed withthe airflow meter 23 during the low opening degree control (i.e.,passing air quantity corresponding to opening degree of intake throttlevalve 19 during low opening degree control) and the opening degree ofthe intake throttle valve 19. Accordingly, the leak air quantity of theintake throttle valve 19 can be calculated with high accuracy.

The air is stored in the intake passage downstream of the intakethrottle valve 19 before the first intake stroke of each cylinder endsafter the engine start is commenced. Therefore, even if the low openingdegree control for controlling the opening degree of the intake throttlevalve 19 to the fully closed position or the proximity of the fullyclosed position is performed during the first intake stroke period sincethe engine start is commenced until the first intake strokes of thecylinders are completed, the air necessary for the combustion in theengine start can be suctioned into the cylinders, inhibiting adverseeffect on the starting performance of the engine 11.

The first embodiment corrects the integration amount ΔI of the ISCfeedback correction amount ISCI in accordance with the leak air quantityof the intake throttle valve 19 during the idle operation to correct theopening degree Thr of the intake throttle valve 19 in accordance withthe leak air quantity of the intake throttle valve 19. Thus, the changein the relationship between the opening degree Thr of the intakethrottle valve 19 and the passing air quantity Gath (changecharacteristic of passing air quantity Gath with respect to openingdegree Thr of intake throttle valve) due to the change in the leak airquantity Qleak of the intake throttle valve 19 is compensated.Accordingly, the controllability of the intake air quantity through theopening degree control of the intake throttle valve 19 can be improvedand the stability of the idle speed can be improved without beingaffected by the aging change of the leak air quantity of the intakethrottle valve 19 and the like.

Furthermore, in the first embodiment, the leak air quantity of theintake throttle valve 19 can be calculated when the engine 11 isstarted. Therefore, the correction of the opening degree of the intakethrottle valve 19 according to the leak air quantity of the intakethrottle valve 19 can be started immediately after the engine start. Asa result, the controllability of the intake air quantity can be improvedeven immediately after the engine start.

Next, a second embodiment of the present invention will be explained inreference to FIGS. 10 and 11. In the second embodiment, a leak airquantity calculation program shown in FIG. 11 is executed to perform lowopening degree control for controlling the opening degree of the intakethrottle valve 19 to the fully closed position or proximity of the fullyclosed position during fuel cut control for suspending the fuelinjection of the engine 11 as shown in a time chart of FIG. 10. Thus,the intake pressure downstream of the intake throttle valve 19 isbrought to pressure that is equal to or lower than the critical pressurewith respect to the intake pressure upstream of the intake throttlevalve 19 and that does not cause oil loss via valve guides in the intakestroke of each cylinder. FC in FIG. 10 represents a fuel cut controlexecution flag. The leak air quantity Qleak at the time when the intakethrottle valve 19 is fully closed is calculated based on the intake airquantity Qafm sensed with the airflow meter 23 during the low openingdegree control.

In the leak air quantity calculation program shown in FIG. 11, S301first determines whether the fuel cut control is in progress. If S301 isYES, the processing proceeds to S302 to set the target opening degreeTThr of the intake throttle valve 19 to the opening degree LThr for thelow opening degree control. The opening degree LThr for the low openingdegree control is the opening degree that brings the intake pressuredownstream of the intake throttle valve 19 to the pressure that is equalto or lower than the critical pressure CP (i.e., pressure at whichpassing air quantity changes in accordance with opening degree of intakethrottle valve 19 without being affected by pressure difference betweenpressure upstream of intake throttle valve 19 and pressure downstream ofthe intake throttle valve 19) with respect to the intake pressureupstream of the intake throttle valve 19 and that is equal to or higherthan lower limit pressure LLP for preventing the oil loss via the valveguides during the intake stroke of each cylinder.

S302 sets the target opening degree TThr of the intake throttle valve 19to the opening degree LThr for the low opening degree control to controlthe actual opening degree Thr of the intake throttle valve 19 to theopening degree LThr for low opening degree control (target openingdegree TThr). Thus, the low opening degree control for controlling theopening degree Thr of the intake throttle valve 19 so that the intakepressure downstream of the intake throttle valve 19 becomes the pressurethat is equal to or lower than the critical pressure CP with respect tothe intake pressure upstream of the intake throttle valve 19 and that isequal to or higher than the lower limit pressure LLP for preventing theoil loss via the valve guides during the intake stroke of each cylinderis performed.

Then, S303 reads the intake air quantity Qafm, the engine rotation speedNe, the coolant temperature Tw, the actual opening degree Thr of theintake throttle valve 19 and the like, which are sensed during the lowopening degree control. Then, with reference to the map of the basicleak air quantity Qleakbse shown in FIG. 8, S304 calculates the basicleak air quantity Qleakbse at the time when the intake throttle valve 19is fully closed in accordance with the intake air quantity Qafm and theactual opening degree Thr of the intake throttle valve 19 at the presenttime (i.e., during low opening degree control). S305 calculates thecorrection coefficient Cne corresponding to the engine rotation speed Neand the coolant temperature Tw with reference to the map of thecorrection coefficient Cne (not shown).

Then, S306 calculates the leak air quantity Qleak at the time when theintake throttle valve 19 is fully closed by multiplying the basic leakair quantity Qleakbse at the time when the intake throttle valve 19 isfully closed by the correction coefficient Cne. Then, S307 updates thelearning value of the leak air quantity Qleak in the learning areacorresponding to the coolant temperature Tw as of the presentcalculation of the leak air quantity Qleak with the presently calculatedleak air quantity Qleak and stores the learning value in the rewritablenonvolatile memory.

The capacity of the intake passage downstream of the intake throttlevalve 19 is small in the system having the intake throttle valves 19 inthe intake manifolds 14 of the respective cylinders of the engine 11.Therefore, if the intake throttle valve 19 is fully closed during thefuel cut control (i.e., when rotation speed of engine 11 is equal to orgreater than predetermined value), there is a possibility that theintake pressure downstream of the intake throttle valve 19 declinesgreatly and the oil loss via the valve guides occurs.

Therefore, the second embodiment performs the low opening degree controlfor controlling the opening degree of the intake throttle valve 19 tothe fully closed position or the proximity of the fully closed positionduring the fuel cut control so that the intake pressure downstream ofthe intake throttle valve 19 becomes the pressure that is equal to orlower than the critical pressure with respect to the intake pressureupstream of the intake throttle valve 19 and that does not cause the oilloss via the valve guides. The leak air quantity at the time when theintake throttle valve 19 is fully closed is calculated based on theintake air quantity sensed with the airflow meter 23 during the lowopening degree control. Accordingly, the leak air quantity of the intakethrottle valve 19 can be calculated with high accuracy while preventingthe oil loss via the valve guides and the adverse effect on theoperation of the engine 11.

In the first and second embodiments, the basic leak air quantityQleakbse is calculated in accordance with the intake air quantity Qafmand the actual opening degree Thr of the intake throttle valve 19 as ofthe low opening degree control. Alternatively, the basic leak airquantity Qleakbse may be calculated in accordance with the intake airquantity Qafm and the target opening degree TThr (i.e., opening degreeLThr for low opening degree control) of the intake throttle valve 19 asof the low opening degree control.

In the first and second embodiments, the integration amount ΔI of theISC feedback correction amount ISCI is corrected in accordance with theleak air quantity of the intake throttle valve 19 during the idleoperation. Alternatively, the opening degree of the intake throttlevalve 19 may be corrected in accordance with the leak air quantity Qleakof the intake throttle valve 19 during normal operation other than theidle operation. Thus, the change in the relationship between the openingdegree Thr of the intake throttle valve 19 and the passing air quantityGath (change characteristic of passing air quantity Gath with respect toopening degree Thr of intake throttle valve) due to the change in theleak air quantity of the intake throttle valve 19 may be compensated.

Next, a third embodiment of the present invention will be explained.First, an outline of an engine intake system will be explained inreference to FIG. 12. The inline four-cylinder engine 11 as an internalcombustion engine has four cylinders of a first cylinder #1 to a fourthcylinder #4. A surge tank 13 is provided to an intake pipe 12 of theengine 11. Intake manifolds 14 for introducing air into respectivecylinders of the engine 11 are provided to the surge tank 13. Intakethrottle valve units 15 are attached to the intake manifolds 14 of therespective cylinders. Injectors (not shown) for injecting fuel areprovided near intake ports of the respective cylinders. Spark plugs (notshown) are attached to a cylinder head of the engine 11 for therespective cylinders. Fuel-air mixture in the cylinders is ignited withspark discharge from the respective spark plugs. An EGR valve 30(exhaust gas recirculation adjustment device) that adjusts an EGRquantity (quantity of exhaust gas recirculation quantity) is provided inan EGR pipe (not shown) for recirculating part of exhaust gas of theengine 11 to the intake air side.

A coolant temperature sensor 25 for sensing coolant temperature Tw and acrank angle sensor 26 for outputting a pulse signal every time acrankshaft of the engine 11 rotates by a predetermined crank angle areattached to a cylinder block of the engine 11. The crank angle CA andengine rotation speed Ne are sensed based on the output signal of thecrank angle sensor 26. Furthermore, an accelerator operation amount ACCP(depressed amount of accelerator) is sensed with an accelerator sensor27.

A variation is caused among the intake air quantities of the cylindersif the leak air quantities of the intake throttle valves 19 provided inthe intake manifolds 14 of the respective cylinders (air quantitiespassing through small gaps between intake throttle valves 19 and intakepassage inner wall faces when intake throttle valves 19 are fullyclosed) vary among the cylinders due to manufacture tolerances, agingchanges, and the like. Therefore, there is a possibility that the torqueof the respective cylinders varies and the engine rotation speedfluctuates largely.

As a countermeasure, the ECU 28 executes an each cylinder intakethrottle valve opening degree correction program shown in FIG. 14 toperform each cylinder intake throttle valve opening degree correctionfor correcting the opening degree of the intake throttle valve 19 inaccordance with the leak air quantity of each cylinder as follows.

If the leak air quantity of the intake throttle valve 19 increases whenthe intake throttle valve 19 has a function to generate an airflowcurrent (e.g., tumble flow or swirl flow) for equalizing the fuel-airmixture, the intensity of the airflow generated by the intake throttlevalve 19 is weakened correspondingly, and the effect to equalize thefuel-air mixture is lowered. Therefore, if the EGR quantity is increasedduring the low load operation of the engine 11, in which the influenceof the EGR is large, as shown by a solid line “a” in a time chart ofFIG. 13, the effect of equalizing the fuel-air mixture is furtherlowered by the influence of the EGR in the cylinder (for example, secondcylinder #2) causing the large leak air quantity of the intake throttlevalve 19. As a result, the combustion state becomes unstable and therotation speed corresponding to the combustion stroke falls greatly overa normal variation range ΔNe(nor). A broken line “b” in FIG. 13 showsthe rotation speed Ne in the case where the airflow control function isnot provided.

Paying attention to such the characteristic, in the present embodiment,first, EGR increase control for controlling the EGR valve 30 to increasethe EGR quantity during the low load operation of the engine 11 isperformed. The rotation fluctuation due to the combustion in therespective cylinders is sensed as a parameter for evaluating thecombustion states of the respective cylinders during the EGR increasecontrol, and the cylinder causing an unstable combustion state isdetermined based on the rotation fluctuation due to the combustion inthe respective cylinders. The cylinder causing the unstable combustionstate is determined to be a large leak air cylinder (cylinder causinglarge leak air quantity).

For example, as shown in the time chart of FIG. 13, the maximum values(peak values) of the rotation speed Ne corresponding to the combustionstrokes of the respective cylinders (first cylinder #1 to fourthcylinder #4) are calculated as the rotation speeds Ne(#1)-Ne(#4) of therespective cylinders #1-#4 during the EGR increase control. Then,average rotation speed Ne(av) of all the cylinders is calculated fromthe rotation speeds Ne(#1)-Ne(#4) of the respective cylinders. Then,deviations between the average rotation speed Ne(av) of all thecylinders and the rotation speeds Ne(#1)-Ne(#4) of the respectivecylinders are calculated. Thus, rotation fluctuation amountsΔNe(#1)-ΔNe(#4) of the respective cylinders toward the lower rotationspeed from the average rotation speed Ne(av) of all the cylinders arecalculated (i.e., ΔNe(#i)=Ne(av)-Ne(#i), i=1 to 4)

Then, the maximum rotation fluctuation amount ΔNe(max) is determined outof the rotation fluctuation amounts ΔNe(#1)-ΔNe(#4) of the respectivecylinders in a predetermined period (period D shown in FIG. 13, forexample, period of 720° CA). If the maximum rotation fluctuation amountΔNe(max) is equal to or greater than a predetermined value α, it isdetermined that the cylinder corresponding to the maximum rotationfluctuation amount ΔNe(max) is the large leak air cylinder. That is, inthe cylinder causing the large leak air quantity, the combustion statebecomes unstable during the EGR increase control, and the rotation speedfalls greatly over the normal variation range ΔNe(nor). Therefore, thecylinder causing the rotation fluctuation amount ΔNe that is directedtoward the lower rotation speed from the average rotation speed Ne(av)of all the cylinders and that is equal to or greater than thepredetermined value α is determined to be the large leak air cylinder.

Then, the opening degree Thr of the intake throttle valve 19 iscorrected in accordance with the maximum rotation fluctuation amountΔNe(max) reflecting the leak air quantity of the intake throttle valve19 of the large leak air cylinder during a period (period E, in FIG. 13)corresponding to the intake stroke of the large leak air cylinder. Thus,the opening degree Thr of the intake throttle valve 19 is corrected inaccordance with the leak air quantity of the intake throttle valve 19 ofthe large leak air cylinder.

By repeatedly performing the processing of sensing the rotationfluctuations of the respective cylinders during the EGR increasecontrol, the processing of determining the large leak air cylinder basedon the rotation fluctuations of the respective cylinders, and theprocessing of correcting the opening degree Thr of the intake throttlevalve 19 in the period corresponding to the intake stroke of the largeleak air cylinder, the variation among the intake air quantities due tothe variation among the leak air quantities of the intake throttlevalves 19 of the respective cylinders is corrected with high accuracy.Thus, the engine rotation fluctuation due to the variation among theleak air quantities of the intake throttle valves 19 of the respectivecylinders is inhibited.

Next, the contents of processing of the each cylinder intake throttlevalve opening degree correction program shown in FIG. 14 executed by theECU 28 will be explained. The each cylinder intake throttle valveopening degree correction program shown in FIG. 14 is executed in apredetermined cycle while the ECU 28 is energized. If the program isstarted, S401 first determines whether the low load operation of theengine 11 is in progress. If S401 is YES, the processing proceeds toS402 to perform the EGR increase control for controlling the EGR valve30 such that the EGR quantity increases.

Then, the processing proceeds to S403 to determine whether the EGRquantities of all the cylinders have increased, for example, based onwhether a predetermined period necessary for the EGR quantities of allthe cylinders to increase has elapsed. If it is determined that the EGRquantities of all the cylinders have increased, the processing proceedsto S404 to calculate the maximum values (peak values) of the rotationspeed Ne corresponding to the combustion strokes of the respectivecylinders as the rotation speeds Ne(#1)-Ne(#4) of the respectivecylinders based on the output of the crank angle sensor 26 during theEGR increase control. Then, the processing proceeds to Step S405 tocalculate the average rotation speed Ne(av) of all the cylinders fromthe rotation speeds Ne(#1)-Ne(#4) of the respective cylinders.

Then, the processing proceeds to S406. S406 calculates the deviationsbetween the average rotation speed Ne(av) of all the cylinders and therotation speeds Ne(#1)-Ne(#4) of the respective cylinders as therotation fluctuation amounts ΔNe(#1)-ΔNe(#4) of the respective cylinderstoward the lower rotation speed from the average rotation speed Ne(av)of all the cylinders (i.e., ΔNe(#i)=Ne(av)-Ne(#i), i=1 to 4).

Then, the processing proceeds to S407 to determine the maximum rotationfluctuation amount ΔNe(max) out of the rotation fluctuation amountsΔNe(#1)-ΔNe(#4) of the respective cylinders in the predetermined period(for example, period of 720° CA). Then, the processing proceeds to S408to determine whether the maximum rotation fluctuation amount ΔNe(max) isequal to or greater than the predetermined value α.

If S408 determines that the maximum rotation fluctuation amount ΔNe(max)is smaller than the predetermined value α, the maximum rotationfluctuation amount ΔNe(max) is within the normal variation rangeΔNe(nor). Therefore, the cylinder corresponding to the maximum rotationfluctuation amount ΔNe(max) is determined not to be the large leak aircylinder, and the program is ended.

If S408 determines that the maximum rotation fluctuation amount ΔNe(max)is equal to or greater than the predetermined value α, the maximumrotation fluctuation amount ΔNe(max) exceeds the normal variation rangeΔNe(nor). In this case, the processing proceeds to S409 to determinethat the cylinder corresponding to the maximum rotation fluctuationamount ΔNe(max) is the large leak air cylinder. Then, the processingproceeds to S410 to correct the opening degree of the intake throttlevalve 19 in the period corresponding to the intake stroke of the largeleak air cylinder in accordance with the maximum rotation fluctuationamount ΔNe(max) reflecting the leak air quantity of the intake throttlevalve 19 of the large leak air cylinder. Thus, the opening degree of theintake throttle valve 19 is corrected in accordance with the leak airquantity of the intake throttle valve 19 of the large leak air cylinder.In this case, the opening degree of the intake throttle valve 19 iscorrected such that the variation among the intake air quantities due tothe variation among the leak air quantities is corrected.

By repeatedly executing the program, the variation among the intake airquantities due to the variation among the leak air quantities of theintake throttle valves 19 of the respective cylinders is corrected withhigh accuracy. As a result, the engine rotation fluctuation due to thevariation among the leak air quantities of the intake throttle valves 19of the respective cylinders is inhibited.

In the above-described third embodiment, attention is paid to thephenomenon that, if the EGR quantity is increased during the low loadoperation of the engine 11, the combustion state in the cylinder causingthe large leak air quantity of the intake throttle valve 19 becomesunstable due to the influence of the EGR and the rotation speedcorresponding to the combustion stroke of the cylinder falls greatly. Inthe third embodiment, the rotation fluctuation amounts ΔNe(#1)-ΔNe(#4)of the respective cylinders toward the lower rotation speed from theaverage rotation speed Ne(av) of all the cylinders are calculated duringthe EGR increase control for increasing the EGR quantity during the lowload operation of the engine. The cylinder corresponding to the maximumrotation fluctuation amount ΔNe(max) is determined to be the large leakair cylinder. The opening degree of the intake throttle valve 19 iscorrected in accordance with the maximum rotation fluctuation amountΔNe(max) reflecting the leak air quantity of the intake throttle valve19 of the large leak air cylinder during the period corresponding to theintake stroke of the large leak air cylinder. Thus, the opening degreeof the intake throttle valve 19 is corrected in accordance with the leakair quantity of the intake throttle valve 19 of the large leak aircylinder.

By repeatedly executing these processings, the variation among theintake air quantities due to the variation among the leak air quantitiesof the intake throttle valves 19 of the respective cylinders can becorrected with high accuracy. Thus, the engine rotation fluctuation dueto the variation among the leak air quantities of the intake throttlevalves 19 of the respective cylinders can be inhibited, and thestability of the idle speed can be improved during the idle operation.

Moreover, there is no need to provide the bypass passages bypassing theintake throttle valves 19 of the respective cylinders or the controlvalves that open and close the bypass passages. Therefore, the systemstructure can be simplified and the cost can be reduced. The presentinvention may be applied to the system having the bypass passagesbypassing the intake throttle valves 19 in the intake manifolds 14 ofthe respective cylinders and the control valves in the bypass passagesof the respective cylinders for opening/closing the bypass passagesrespectively.

The maximum rotation fluctuation amount ΔNe(max) exceeds the normalvariation range ΔNe(nor) when the maximum rotation fluctuation amountΔNe(max) is greater than the predetermined value α. Therefore, in thepresent embodiment, it is determined that the cylinder corresponding tothe maximum rotation fluctuation amount ΔNe(max) is the large leak aircylinder. When the maximum rotation fluctuation amount ΔNe(max) issmaller than the predetermined value α, the maximum rotation fluctuationamount ΔNe(max) is within the normal variation range ΔNe(nor). In thiscase, it is determined that the cylinder corresponding to the maximumrotation fluctuation amount ΔNe(max) is not the large leak air cylinder.Thus, erroneous determination that the cylinder causing the rotationspeed slightly lower than the average rotation speed is the large leakair cylinder can be precluded.

In the third embodiment, it is determined that the cylinder causing thelargest rotation fluctuation amount toward the lower rotation speed fromthe average rotation speed of all the cylinders during the EGR increasecontrol is the large leak air cylinder. Alternatively, it may bedetermined that the cylinder causing the largest rotation fluctuationamount between the time before the EGR increase control is performed andthe time when the EGR increase control is performed is the large leakair cylinder.

In the third embodiment, the EGR increase control for increasing theexternal EGR quantity by controlling the EGR valve 30 is performed. In asystem having a variable valve timing device that changes valve timingof the intake valve or the exhaust valve, the EGR increase control forincreasing an internal EGR quantity by controlling a valve overlapamount between the intake valve and the exhaust valve may be performed.

In the first to third embodiments, the present invention is applied tothe four-cylinder engine. Alternatively, the present invention may beapplied to a two-cylinder engine, a three-cylinder engine, or an enginehaving five or more cylinders.

In the first to third embodiments, the present invention is applied tothe intake port injection engine. Alternatively, the present inventionmay be applied to a direct injection engine or a dual injection enginehaving injectors in both of the intake port and the cylinder.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not to be limited to thedisclosed embodiments, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1. A controller of an internal combustion engine having intake throttlevalves in branch intake passages of respective cylinders for regulatingintake air quantities, the branch intake passages branching from a mainintake passage of the engine for introducing the intake air into therespective cylinders, the controller comprising: an intake air quantitysensor provided in the main intake passage for sensing the intake airquantity; a low opening degree control device that performs low openingdegree control for controlling an opening degree of the intake throttlevalve during fuel cut control for stopping fuel injection of the enginesuch that intake pressure downstream of the intake throttle valvebecomes pressure that is equal to or lower than predetermined criticalpressure with respect to intake pressure upstream of the intake throttlevalve and that does not cause oil loss via valve guides during theintake stroke of each cylinder; a leak air quantity calculation devicethat calculates a leak air quantity at the time when the intake throttlevalve is fully closed based on the intake air quantity sensed with theintake air quantity sensor during the low opening degree control; and anintake throttle valve opening degree correction device that corrects theopening degree of the intake throttle valve in accordance with the leakair quantity.
 2. The controller as in claim 1, further comprising: anidle speed control device that performs idle speed control forperforming feedback control of the opening degree of the intake throttlevalve such that actual rotation speed of the engine coincides withtarget idle speed during idle operation of the engine, wherein theintake throttle valve opening degree correction device corrects afeedback gain of the idle speed control in accordance with the leak airquantity during the idle speed control.
 3. A controller of an internalcombustion engine having intake throttle valves in intake passages ofrespective cylinders of the engine for regulating intake air quantities,the intake throttle valve having a function to generate an airflow forequalizing a fuel-air mixture, the controller comprising: an exhaust gasrecirculation adjustment device that adjusts an exhaust gasrecirculation quantity of the engine; an exhaust gas recirculationincrease control device that performs exhaust gas recirculation increasecontrol for controlling the exhaust gas recirculation adjustment devicesuch that the exhaust gas recirculation quantity increases during lowload operation of the engine; an each cylinder leak air quantityinformation sensing device that senses a combustion state in eachcylinder during the exhaust gas recirculation increase control asinformation about a leak air quantity at the time when the intakethrottle valve of each cylinder is fully closed; a large leak aircylinder determination device that determines a cylinder causing a largeleak air quantity of the intake throttle valve to be a large leak aircylinder based on the combustion state of each cylinder sensed with theeach cylinder leak air quantity information sensing device; and an eachcylinder intake throttle valve opening degree correction device thatcorrects an opening degree of the intake throttle valve during a periodcorresponding to an intake stroke of the large leak air cylinder inaccordance with the combustion state in the large leak air cylinder. 4.The controller as in claim 3, wherein the each cylinder leak airquantity information sensing device senses a rotation fluctuation of theengine due to the combustion in each cylinder as a parameter forevaluating the combustion state in each cylinder; and the large leak aircylinder determination device determines the cylinder causing theunstable combustion state to be the large leak air cylinder based on therotation fluctuation due to the combustion in each cylinder.
 5. Thecontroller as in claim 4, wherein the each cylinder leak air quantityinformation sensing device senses the rotation fluctuation due to thecombustion in each cylinder by comparing rotation speed of the enginecorresponding to a combustion stroke of the cylinder with averagerotation speed of all the cylinders, and the large leak air cylinderdetermination device determines the cylinder causing the largestrotation fluctuation amount toward lower rotation speed from the averagerotation speed to be the large leak air cylinder.
 6. The controller asin claim 5, wherein the large leak air cylinder determination devicedetermines the cylinder causing the largest rotation fluctuation amountthat is directed toward lower rotation speed from the average rotationspeed and that is equal to or greater than a predetermined value to bethe large leak air cylinder.